Audience groups are designed to be contained within a sub account. When audience members agree to receive emails from you (by signing up or otherwise actively interacting and giving permission), they expect to receive email from the folks they gave permission to.

There is not a way to share audience members between accounts, but if your parent account is set up such that the audience members have given permission to receive mailings from each of your subs, then you can upload lists into more than one sub.